Maximal-aperture reflecting objective

ABSTRACT

Objectives and other optical assemblies include a reflective surface that is truncated at or near a focus based on a curvature of the reflective surface. A specimen is situated at or near the focus of the reflective surface, so that the reflective surface captures and collimates optical radiation emitted from the specimen. The reflective surface can be defined on an optical substrate along with a lens surface, so that an illumination flux is focused on the specimen by the lens surface, and a secondary light flux produced in response to the illumination flux is captured and collimated by the reflective surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application60/723,554, filed Oct. 3, 2005 that is incorporated herein by reference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with United States government support pursuantto grant no. MH068481 from the National Institutes of Health, and theUnited States government has certain rights in the invention.

TECHNICAL FIELD

The disclosure pertains to microscope objectives and microscopy methods.

BACKGROUND

Advances in microscopy have permitted increasingly sophisticatedinvestigations of biological and other systems. In traditionalmicroscopy, a specimen is illuminated with a broadband (“white”) lightsource, and a magnified image of the specimen is produced. Resolutionand image contrast in such systems generally depend on specimen spectralabsorbance and microscope objective numerical aperture and aberrations.Microscopy can also be based on radiation produced in a specimen inresponse to a exposure to a suitable light flux. For example, influorescence microscopy, fluorescence produced in a specimen in responseto a stimulating light flux is used to form a specimen image. In otherexamples, a specimen can emit a light flux based on a stimulating lightflux via a multiphoton process. In both fluorescence microscopy andmultiphoton microscopy the emitted light flux can be used to produce animage directly, or the emitted light flux can be localized by scanningthe stimulating light flux across the specimen.

While microscopy-based specimen analysis based on fluorescence ormultiphoton processes can provide significant specimen information, suchanalysis is typically hampered by inefficiencies in the fluorescencecollection and multiphoton excitation processes. In some investigations,low efficiencies can be compensated for by increasing the time duringwhich the specimen is exposed to the stimulating light flux, resultingin longer measurement times. In other examples, the stimulating lightflux produces specimen changes so that increased exposures cannot beused. Therefore, improved microscopy methods and apparatus are needed.

SUMMARY

According to representative examples, optical elements comprise aconcave reflector situated on an optical axis, wherein the concavereflector has a reflective surface and is truncated at a sample surface.The concave reflector is configured to direct a light flux received froma sample situated optically proximate the sample surface to a detectionregion. In illustrative examples, the sample surface is substantiallyplanar and the reflective surface is defined on an exterior surface of atransparent optical substrate. In additional examples, the transparentoptical substrate is configured to define a refractive optical surfacethat is situated on the optical axis and is configured to converge anincident light flux toward the sample surface. In other examples, acurvature of the reflective surface is aspheric such as, for example, aparabola or other conic section. In typical examples, the sample surfaceis situated optically proximate a focus of the reflective opticalsurface. In further examples, the reflective optical surface extends soas to receive a light flux over solid angles of at least about π, 1.5π,1.7π, or 1.9π steradians. In other examples, the reflective opticalsurface includes a probe aperture that defines a probe path that couplesthe sample surface and the probe aperture.

According to other aspects of the technology, optical assembliescomprise a lens situated on an optical axis and configured to receive aprimary light flux and converge the primary light flux toward a sampleplane. A concave reflective surface is situated along the optical axiswith respect to the sample plane so as to direct a secondary light fluxproduced in response to the primary light flux back towards the lens andalong the optical axis. In some particular examples, the lenssubstantially occupies an exit aperture of the reflective surface andthe reflective surface is configured to substantially collimate thesecondary light flux.

Catadioptric objectives comprise a transparent refractive substratehaving a refractive surface configured to direct an input optical beamtoward a target region and a reflective surface configured to receive atleast a portion of a secondary light flux propagating from the targetregion produced in response to the input optical beam. In some examples,the reflective surface includes a reflective coating and is a surface ofrotation. According to other examples, the refractive surface issituated on an optical axis and has a positive curvature with respect tothe input beam, and the reflective surface is situated on the axis andhas a negative curvature with respect to the input beam. In otherrepresentative examples, the reflective surface is an aspheric surfacesuch as a parabola or other conic section. In additional examples, acenter of rotation of the refractive surface and a center of rotation ofthe reflective surface are situated on an optical axis. In otherexamples, an exit surface truncates the reflective surface and issituated substantially at a focus of the refractive surface. In otherexamples, an exit surface truncates the reflective surface and issituated substantially at a focus of the reflective surface.

Transparent optical substrates comprise, along an axis, an input surfacedefining an input aperture and configured to receive a light flux and anexit surface defining an exit aperture configured to receive the lightflux from the input aperture. A reflective surface extends from theinput surface to the exit surface, and the exit surface is situatedoptically proximate a focus of the reflective surface. In some examples,the input and exit surfaces are substantially planar, or the inputsurface is configured to increase a convergence of the received lightflux. In other examples, an optical coating is situated at thereflective surface and is configured to enhance a reflectivity of thereflective surface.

Immersion objectives comprise a reflective surface having an inputaperture configured to receive a primary light flux and an exit apertureconfigured to deliver the primary light flux to a sample. The reflectivesurface, the input aperture, and the exit aperture are situated on anoptical axis, and an internal volume is defined by the input aperture,the exit aperture, and the reflective surface. An optically transmissivebarrier is coupled to the reflective surface and configured to dividethe internal volume into an illumination volume and an immersion volume.According to additional examples, the exit aperture is situated at afocus of the reflective surface. In still further examples, a hollowshell defines the input aperture, the exit aperture, and the reflectivesurface. In representative examples, the immersion volume is filled withan immersion fluid and a portion of the reflective surface that boundsthe internal volume is defined on an optically transmissive substrate.In additional examples, a lens is situated at the optically transmissivebarrier and configured to converge a substantially collimated light fluxincident from the input aperture at the exit aperture. In stilladditional examples, the reflective surface includes a probe aperture ina portion of the reflective surface that defines the immersion volume,wherein the probe aperture is coupled to the immersion volume. In othertypical examples, a probe aperture axis extending from the probeaperture intersects the exit aperture substantially at the optical axis,wherein an angle between the probe axis and the optical axis is lessthan about 60 degrees.

These and other aspects and features of the disclosed technology aredescribed below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a portion of a microscopy system.

FIGS. 2-3 are schematic diagrams illustrating light collection solidangles associated with microscopy systems such as illustrated in FIG. 1.

FIG. 4 is a schematic diagram of another example of a microscopy system.

FIG. 5 is a schematic diagram of a reflector having an interior volumethat has a portion that is filled with an immersion liquid.

FIG. 6 is a schematic diagram of a reflector that includes a probeaperture.

FIG. 7 is a schematic diagram of an objective defined by a transparentsubstrate.

FIG. 8 is a schematic diagram of an objective and an electrode situatedfor specimen analysis.

FIG. 9 is a schematic diagram of a microscope system that includes atruncated reflector situated so that a sample is situated at or near afocus of a reflective surface of the truncated reflector.

FIG. 10 is a schematic diagram of a portion of the microscope systemillustrated in FIG. 9.

FIG. 11 is a schematic diagram of a reflective optical system thatincludes an objective based on a Cassegrain configuration that isadapted to direct an incident light flux to an image plane forillumination of a specimen.

FIG. 12 is a schematic diagram that includes a microscope objective fordirecting an illumination beam to a target, and a reflector configuredto direct light from the target to a detector.

FIG. 13 is a schematic diagram that illustrates a glass terminationconfigured to direct a light flux emitted or scattered by a target to adetector.

FIG. 14 is a schematic diagram of an optical system that includes adeformable mirror.

FIG. 15 is a schematic diagram of a collection optical system thatincludes first and a second windows configured to probe access to aspecimen.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”While particular examples and applications for the disclosed embodimentsare also disclosed, the described systems, methods, and apparatus shouldnot be construed as limiting in any way. Instead, the present disclosureis directed toward all novel and nonobvious features, aspects, andequivalents of the various disclosed embodiments, alone and in variouscombinations and sub-combinations with one another. The disclosedsystems, methods, and apparatus are not limited to any specific aspect,feature, or combination thereof, nor do the disclosed systems, methods,and apparatus require that any one or more specific advantages bepresent or problems be solved. In addition, the attached figures may notshow the various ways in which the disclosed systems, methods, andapparatus can be used in conjunction with other systems, methods, andapparatus, and components that are well known in the art are notdescribed in detail.

Referring to FIG. 1, a portion of a microscopy system includes anillumination source 102 configured to produce an illumination beam 104that is directed to a specimen 110 by a reflector 106 and a lens 108situated on an axis 103. The reflector 106 can be a front surface orrear surface reflector that includes a metallic layer, a dielectriclayer, or a multilayer coating of metallic and/or dielectric layers. Thelayers can be selected to obtain a selected reflectance in a particularwavelength range so that the reflectance and transmittance of thereflector 106 can be configured as convenient for other wavelengths andwavelength ranges. Alternatively, the reflector 104 can be implementedas a prism, or otherwise be based on total internal reflection. Asdiscussed below, the reflector 106 can be configured to have opticalpower by, for example, providing a reflective surface on a concave orconvex mirror, or using a holographic element with optical power, withor without spectral selectivity.

The lens 108 is configured to converge the illumination beam on thespecimen 110. In some examples, the illumination beam is substantiallycollimated, and the lens 108 focuses the illumination beam onto thesample 110. In other examples, the illumination beam is diverging orconverging and the lens 108 is selected so that the illumination beamdivergence is increased or decreased as convenient (for an incidentconverging illumination beam) or becomes converging (for an incidentdiverging illumination beam). Thus, the lens 108 can have eitherpositive or negative optical power. In other examples, the illuminationbeam can be appropriately converging prior to the lens 108, and the lens108 can be omitted or replaced with an optical element having littleoptical power. For example, the reflector 106 can be configured as aconcave mirror that converges the illumination beam toward the sample110 so that the lens 108 becomes unnecessary.

For convenience in illustration, the lens 108 is shown as a single lenselement (“siinglet”), although in many typical examples, the lens 108 isa compound lens that includes two or more optical elements such as foundin, for example, microscope objectives or other compound lenses. Whilelenses are particularly convenient, in additional examples, a curvedmirror or combination of mirrors and lenses can be used to focus ordirect the illumination beam 104. For example, reflecting microscopeobjectives based on the Schwarzchild objective or on a Cassegrainreflector configuration can be used. The Schwarzchild objective isconfigured to convert one image plane into another image plane, whileCassegrain reflector configurations have infinite conjugates. Thus, someadjustment is preferred to accommodate the infinite conjugate to imageplane focusing applied to the illumination beam 104.

The illumination beam 104 is generally selected to produce emittedradiation at the specimen 110 in association with an intendedmeasurement or specimen characterization. For example, in a fluorescencemeasurement, a wavelength range of the illumination beam is selected toinduce fluorescence in the specimen 110 or in fluorophores provided byor associated with the specimen 110. Alternatively, for non-fluorescencebased valuations based on, for example, light scattering, a wavelengthrange of the illumination beam can be selected to enhance lightscattering. In multiphoton measurements, illumination beam optical powercan be selected as well. To enhance production of multiphoton-basedradiation, peak optical powers can be selected. For convenience indescribing embodiments of the technology, optical radiation emitted byor otherwise emanating from the specimen 110 is referred to as exitingradiation or “secondary radiation.” The illumination beam that producesthis secondary radiation can be referred to as “primary” radiation.” Insome examples, this exiting radiation is wavelength shifted as a resultof fluorescence or a multiphoton process, and such exiting radiation isaccordingly referred to as wavelength shifted. In some applications,primary radiation is generally in a portion of the electromagneticspectrum from about 200 nm to about 2000 nm, but primary radiation canbe at other wavelengths as well. For example, primary radiation can beat soft x-ray wavelengths, extreme ultraviolet wavelengths, or atinfrared or far infrared wavelengths. Wavelengths between at least about1 nm and 20 μm can be used, and all reflecting designs permit the use ofwavelengths at which refractive materials are unsuitable. In reflectivesystems, surface reflectivity can be enhanced with multilayer or othercoatings.

As shown in FIG. 1, the exiting radiation propagating alongrepresentative rays 112, 114, 116, 118 is incident to a reflectivesurface 120 of a reflector assembly 122. The surface 120 is configuredto direct the exiting radiation along the axis 103 and to converge,collimate, or at least partially converge or collimate the exitingradiation to form an exit beam 124. The exit beam 124 is received by alens 126 and further converged, diverged, focused or otherwise processedfor delivery to a detector or detector array, or additional processingoptical elements.

The reflective surface 120 can be conveniently selected as a parabolicsurface or to substantially approximate a parabolic surface. For such asurface, the reflector assembly can be truncated at a truncation plane111 so that the specimen 110 (or a selected region of interest in thespecimen) can be situated substantially at a focus of the parabolicsurface. In this way, the exiting radiation is substantially collimatedby the reflective surface 120. In addition, situating the specimen 110at or near a focus permits the reflective surface 120 to capture exitingradiation over a large solid angle. As shown in the schematic diagram ofFIG. 1, the lens 108 and the reflector 106 are the only potentiallimitations on collection of exiting radiation on an illumination beamside of the specimen 110. The reflector 106 can be configured tosubstantially transmit exiting radiation by using, for example, amultilayer dielectric reflective layer or a holographic reflector sothat reflection at the reflector 106 is based on wavelength or angle ofincidence or both. The lens 108 can be configured to substantiallycollimate a portion of the exiting radiation incident to the lens 108,so that, in combination, the reflective surface 120 and the lens 108direct exiting radiation from substantially all of the solid angle onthe illumination beam side of the specimen 110 into the exit beam 124.

The reflective surface 120 can be conveniently selected to be parabolic,elliptical, hyperbolic, spherical, or a generalized aspheric surface.The reflective surface 120 need not be rotationally symmetric, andcylindrical surfaces including surfaces having different curvaturesalong different axes can be used, wherein the curvatures can be selectedto be parabolic, elliptic, hyperbolic, spherical, or otherwise aspheric.

In some examples, imaging of the specimen 110 is achieved by scanningthe illumination light flux across the specimen 110. Exiting light fluxassociated with some or all specimen locations can be collected and usedto provide a specimen image. Typically a laser or other collimated lightsource provides the illumination light flux, and varying a tilt angle ofthe reflector 106 can scan the converging or focused beam across thespecimen, but other scanning methods can also be used. For example,acousto-optic scanners can be used. In applications in which theillumination light flux is scanned, image resolution can be based on thedetected exiting radiation from some or all locations, so that imagingrequirements imposed on the reflective surface 120 can be modest, andthe reflective surface 120 can exhibit significant aberrations withoutadversely affecting image resolution.

FIGS. 2-3 are schematic diagrams illustrating light gathering aperturesavailable in representative examples. Referring to FIG. 2, a reflectivesurface 202 is configured to receive exiting light propagating along,for example, a representative ray 204 such that, after reflection, theray 204 is substantially parallel to an axis 206. In FIG. 2, thereflective surface 202 is configured to substantially collimate theexiting radiation but, as noted above, different collimationconfigurations can be selected. As is apparent from FIG. 2, exitingradiation is collected and directed by the reflective surface incooperation with a lens such as the lens 108 within a cone of half angleθ. The solid angle subtended by such a cone is 2π[1−cos(θ/2)] sosubstantially all exiting radiation is captured. In representativeexamples, the half angle θ is at least about 60 degrees, 75 degrees, 80degrees, or 85 degrees.

Referring to FIG. 3, a reflective surface 302 is configured to collimatean exiting light flux received from a specimen 304 so as to propagatealong an axis 306. A lens 308 that is typically provided to direct anillumination flux to the specimen 304 receives a portion of the exitingradiation that is directed into a cone of angle θ_(r). Such radiationdoes not intersect the reflective surface 302. The lens 308 isconfigured to collimate or substantially collimate this portion of theexiting radiation, so that the lens 308 is not associated with anyappreciable reduction in exiting radiation exiting along the axis 306.

With reference to FIG. 4, an illumination light flux shown as rays 402,404 propagates along an axis 406 through a beamsplitter 408 and to afocusing lens 410. The beamsplitter 408 is configured to substantiallytransmit the illumination light flux and reflect an exiting light flux,such as, for example, a wavelength shifted light flux associated with,for example, fluorescence or a multiphoton process. The illuminationlight flux is focused or otherwise converged by the lens 410 and isincident to a target 412. A specimen of interest can be convenientlysituated on or near a surface 413 of the target 412. Exiting radiation(such as, for example, fluorescence) is reflected by a reflectivesurface 414 and propagates along example rays 416, 418 to the lens 410.The lens 410 can increase exiting radiation beam convergence, and theexiting radiation can be reflected by the beamsplitter 408 to a detector420 or additional optics for further processing. In the example of FIG.4, the lens 410 is configured to direct an illumination light flux tothe target 412 and to direct (in cooperation with the reflective surface414) the exiting radiation to the detector 420.

With reference to FIG. 5, a reflective objective 500 includes areflective surface 502 and a lens 504. An interior volume 506 defined bythe reflective surface 502 and a window 508 can be filled with a highrefractive index material such as an immersion oil (n=1.56) or water,and the lens 504 can be attached to or otherwise situated in the volume506 by the window 508. The reflective surface 502 terminates at a sampleplane 512, and includes an aperture 514 that permits fluid entry into avolume 518. A portion 516 of the reflective surface 502 situated betweenthe window 508 and the sample plane 512 can be defined as a continuationof a portion of the reflective surface defining the volume 506, but insome examples, this portion can be configured for mechanical support, toprovide a suitable spacing of the lens 504, the sample plane 512, and afocal point or focal plane of the reflective surface 502, or otherwiseconfigured. In this alternative, any exiting radiation from the sampleplane 512 incident to the reflector portion 516 may be lost for furtherprocessing. As shown in FIG. 5, perfusion fluid 520 fills the volume 518and at least partially covers a specimen support 524. FIG. 6 is aschematic diagram of a reflective objective similar to that of FIG. 5but that includes an aperture 602 configured to receive an electrode, apatch pipette, or other specimen probe to permit additional specimenmanipulation, stimulation, or measurement.

With reference to FIG. 7, an objective 700 that is of unitary one-piececonstruction includes a reflective surface 704 and a lens surface 702that are defined in a single optically transmission substrate 706.Curvature of the reflective surface 704 is defined by the substrate 706,and a dielectric or metallic coating or a holographic layer can beprovided to enhance or spectrally configure reflectance. The substrate706 is truncated at a sample plane 710, and a specimen support 714 isoptically contacted to a termination 716 of the substrate 706 with aliquid 718 such as an immersion oil, water, or other liquid.Alternatively, an air gap can be provided. As shown in FIG. 7,representative sample portions 720 such as cells are situated on asurface of the specimen support 714. In this example, a single substrateprovides convergence of an illumination light flux, andcollection/convergence of exiting radiation.

Referring to FIG. 8, a unitary objective 802 includes a transmissivesubstrate 801 having a reflective surface 804 and a refractive surface806. The refractive surface 806 is configured to direct an illuminationflux incident along representative rays 808, 810 to a specimen support812, such as a microscope slide or cover slip, and, in cooperation withthe reflective surface 806, gathers and converges or otherwise deliversexiting radiation to a detector or additional optical elements forprocessing. A coating can be provided to enhance reflectivity. Thesubstrate 801 is truncated at the specimen support based on a curvatureof the reflective surface 806 so that exiting radiation is collected,collimated, or converged as needed for a particular application.Typically, truncation is at or near a focus of the reflective surface806. The substrate 801 also includes a relief zone 820 or apertureconfigured to permit access to the specimen support 812 by an electrode814 or other probe.

FIGS. 9-10 illustrate a representative example in which a truncatedreflective surface 912 of a reflective light collector 913 and aconventional microscope objective 902 are configured so that anillumination light flux and an exiting light flux are situated onopposite sides of a specimen. The microscope objective 902 contacts anperfusion fluid layer 904 that covers or partially covers a specimen 904such as a tissue layer that is supported by, for example, a chamber wallor microscope slide 908. An interior volume 916, defined by thereflective surface 912, an entrance window 909, and an exit window 914can be filled with an immersion oil or other material. In this example,the light collector 913 can be substituted for a conventional microscopecondenser. The entrance window 909 and the microscope slide 908 can beoptically contacted with a layer 920 of immersion oil or other material.

FIG. 11 illustrates an all-reflecting optical system. Reflectivesurfaces 1103, 1105 or respective reflectors 1102, 1104 receive anincident light flux 1101 and direct the incident light flux 1101 to aregion of interest 1112 on, for example, a tissue specimen 1108.Curvatures of the reflective surfaces 1103, 1105 can be selected so thatthe incident light flux 1101 can be focused to a nearlydiffraction-limited spot at the specimen 1108: A non-imaging reflector1106 receives light scattered or emitted in response to the illuminationflux at a reflective surface 1107, and directs this light to a detectoror other optical system (not shown in FIG. 11). For example, scatteredor emitted light directed along representative ray directions 1114,1116, 1118 can be focused, reflected, or otherwise processed fordelivery to a detector or other optical system.

As shown in FIG. 11, the reflectors 1102, 1104, 1106 are rotationallysymmetric about an optical axis 1120, but in other examples, asymmetric,tilted, or other surfaces can be used. Reflective surfaces can beconveniently provided by metallic, dielectric, holographic, or othercoatings. In some examples, one or more spectrally selective coatingscan be used. The reflective surfaces 1103, 1105, 1107 can be selected tobe substantially spherical, elliptical, parabolic, or hyperbolic, or canbe aspherical and described based on a polynomial expansion.

Representative reflector specifications are provided in the table below.As used in this table, a mirror surface can be described as a surfacesag X as a function of surface height Y (typically distance from anaxis) such that${X = {\frac{{cY}^{2}}{1 + \sqrt{\left\lbrack {1 - {c^{2}{Y^{2}\left( {1 - e^{2}} \right)}}} \right\rbrack}} + {a^{4}Y^{4}} + {a^{6}Y^{6}} + {a^{8}Y^{8}} + {a^{10}Y^{10}}}},$wherein c is a spherical curvature, e is an eccentricity, and α₄, α₆ α₈α₁₀ are aspheric coefficients. For convenience, the term −e² is referredto as the conic constant, and abbreviated CC herein. This representationof a surface is described in detail in, for example, Rudolf Kingslake,Lens Design Fundamentals, Academic Press, New York (1978) that isincorporated herein by reference. A mirror specification d representsaxial distance between surfaces. For example, optical surface separationalong an axis.

Specifications r_(min), and r_(max) represent apertures of sufficientsize to collect and deliver selected portions of an illumination lightflux or a light flux emitted or scattered in response. For example, thereflective surface 1103 extends from an optical axis OA to a radialdistance of 2.9 mm. The reflective surface 1105 has a central aperturefor light entrance and exit of radius 10.5 mm and an overall radius of42 mm. These dimensions are dependent on numerical aperture, anddifferent values can be used. Optical Specifications for FIG. 11. (Alldimensions in mm.) Surf. c CC a₄ a₆ a₈ a₁₀ d r_(min) r_(max) 1103 6.50.876 −2.857 · 10⁻⁴ −7.594 · 10⁻⁶  −2.241 · 10⁻⁷ −1.442 · 10⁻⁸  −45 2.91105 54.648 1.3 · 10⁻⁴ −1.828 · 10⁻⁹ −3.967 · 10⁻¹³   1.678 · 10⁻¹⁷−6.175 · 10⁻²⁰ 66.0483 10.5 42.0 1107 −5.988 −0.93 −3.0483 6.9 13.5

As summarized in the table above, the reflective surface 1107 is almostparabolic with a conic constant of −0.93 (a parabola has a conicconstant of −1). The configuration of FIG. 11 is all-reflective andintroduces no additional temporal dispersion so that it is particularlyadapted for use with femtosecond or other short pulse illumination. Inaddition, as shown in FIG. 11, the reflector 1106 is displaced along theaxis 1120 for the specimen by a working distance d_(w) that is typicallyconveniently selected to be about 1 mm. The effective (immersed)numerical aperture of the illumination system (reflectors 1102, 1104) isabout 0.86 (0.65 in air) at a magnification of about 30, and a typicalfield of view has a diameter of about 200 μm.

In typical applications, a scattered or secondary light flux appears tooriginate at a location different from its true origin. As a result,superior collection efficiency can be achieved by a relativedisplacement of the reflectors 1102, 1104 and the reflector 1106 alongthe axis 1120. Typically, the reflector 1106 can be configured toreceive a light flux that appears to originate within a region extendingabout one scattering length deep into the specimen 1108. In thisconfiguration, a focus for the illumination light flux can be locatedfurther within the specimen than an apparent origin of the secondaryradiation, and an effective focus of the reflector 1106 can be displacedfrom that of the reflectors 1102, 1104. However, with a specimen that isa thin turbid slab with a reflector provided on an exit surface of theslab, opposite focal displacements can be used (i.e., the source ofsecondary radiation is deeper in the specimen than a reflected focus ofthe illumination reflectors). Thus, collection efficiencies can beimproved with displacement of the effective foci of illumination andcollection optics.

FIG. 12 illustrates a refracting objective 1202 configured to deliver anillumination flux 1201 to a region of interest (ROI) 1210 of a tissuespecimen 1208. Scattered or emitted light from the ROI 1210 directedalong, for example, representative ray directions 1212, 1214, isreflected by a non-imaging reflector 1204 for delivery to a detector(not shown in FIG. 12).

FIG. 13 illustrates an arrangement that can provide increased collectionof a scattered or emitted light flux such as that produced by, forexample, epifluorescence. A collection reflector 1312 and an immersionelement 1302 are situated along an axis 1300 and are configured toreceive a light flux emitted by or scattered from a specimen 1308. Theimmersion element has a surface 1304 that is adapted to contact animmersion fluid layer 1310 that is situated on a surface of the specimen1308. The immersion fluid layer 1310 can be an immersion oil layer orother liquid layer, or the immersion fluid can be omitted and an air gapprovided between the surface 1304 and the specimen 1308, but lightcollection efficiency is generally substantially improved using animmersion fluid layer. The immersion element 1302 also includes a sidesurface 1306 that is configured to have a relatively high reflectivityfor a light flux received from the specimen 1308. High reflectivity canbe provided by total internal reflection or with a dielectric, metallic,or other coating. Such coatings can be broadband coatings, or can beselected to provide a high reflectance only at wavelengths associatedwith, for example, epifluorescence.

The immersion element 1302 is conveniently made of glass, fused silica,or other material that is substantially transparent in a region ofinterest. In a typical example, the immersion element 1302 isrotationally symmetric about the axis 1300. The side surface 1306 can beconfigured to correspond to sides of a cone so that as projected into aplane containing the axis 1300, the side surface 1302 appears as astraight line. The side surface 1302 can also be selected to correspondto a portion of a spherical, elliptical, parabolic or other asphericsurface. In other examples, the immersion element 1302 can include oneor more edge facets instead of a signal continuous side surface asillustrated in FIG. 13.

As shown in FIG. 13, a portion of a light flux from the specimen 1308that is directed along a ray direction 1316 is transmitted by theimmersion element 1302 and reflected by the collection reflector 1312 toa detector or other optical system (not shown in FIG. 13). A portion ofthe light flux propagating along a ray direction 1314 is reflected bythe side surface 1306 and can be directed to a detector withoutreflection by the collection reflector 1312.

FIG. 14 illustrates a representative optical system that includesreflectors 1402, 1406 that are configured to deliver an illuminationflux along an axis 1400. An incident light flux propagating along theaxis 1400 is transmitted by an aperture 1404 to a deformable reflectivesurface 1407 of the reflector 1406. The deformable reflective surface1407 directs the light flux to a reflective surface 1403 of thereflector 1402 that reflects the light flux to a specimen 1412 through awindow 1409 and an immersion fluid 1408. Fluorescence or other emittedor scattered light from the specimen 1412 can be directed by a reflector1414 to another optical system or a detector. The window 1409 can beconveniently secured to the reflector 1414.

In many practical examples, the incident light flux propagates into thespecimen 1412 in order to produce fluorescence or other scattered oremitted light flux that is to be used for specimen investigation. Thespecimen 1412 can introduce significant optical aberrations into theincident light flux. Such aberrations can be reduced or eliminated byadjusting a curvature of the deformable reflective layer 1407 based onan input from a control system 1416, or otherwise adapt a curvature orphase of an illumination flux.

An alternative termination of a fluorescence collection reflector 1502is illustrated in FIG. 15. A first window 1504 and a second window 1506are secured to a specimen side 1503 of the collection reflector 1502 andare configured to contact a immersion or perfusion fluid layer situatedon a specimen 1510. The first window 1504 and the second window 1506join at an intersection 1507, and the second window 1506 is oriented toprovide access to the specimen 1510 by, for example, a patch pipette1512. In addition, a portion of the reflector 1502 is removed to permitaccess to the specimen.

A clear aperture of the first window 1504 can be selected fortransmission of marginal rays 1516, 1518 associated with an illuminationlight flux. If the intersection 1507 is positioned in this way, theillumination light flux is substantially transmitted by the first window1504. In other examples, portions of the illumination light flux can betransmitted by both the first window 1504 and the second window 1506.

The preceding examples are representative of some principles of thedisclosed technology. In some examples, incident and exiting opticalbeams are referred to as being focused by one or more optical elements.While in some examples, a beam focus can be associated with a positionat which an at least approximately minimum beam diameter is established,in other examples a beam focus more broadly encompasses a position orplane at which a beam is sufficiently converged to have a selected beamdiameter, not necessarily a minimum beam diameter. Converging opticalelements such as lenses having positive powers generally have anassociated focus or focal point at which a collimated beam is focused. Abeam propagating parallel to an axis of such an element is generallyconverged to a focal point. Typically a focal point is an on-axis pointin an associated focal plane. In some examples, optical elements canlack well defined focal planes due to, for example, focusingaberrations, but a plane of “best” focus can be defined, and such aplane may or may not include the on-axis focal point.

In the examples, exiting radiation can be collected over substantialsolid angles, ranging up to solid angles that are approximately 2πsteradians. In examples in which front side and back side lightcollection is practical, collection angles can approach 4π steradians byplacing collection optics on both sides of a specimen. It will beapparent that these examples can be rearranged without departing fromthe disclosed principles. Accordingly, the examples are not to be takenas limiting in any way. We claim all that is encompassed both literallyand by equivalents to the appended claims.

1. An optical element, comprising a concave reflector situated on anoptical axis, the concave reflector having a reflective surface andbeing truncated at a sample surface, wherein the concave reflector isconfigured to direct a light flux received from a sample situatedoptically proximate the sample surface to a detection region.
 2. Theoptical element of claim 1, wherein the sample surface is substantiallyplanar.
 3. The optical element of claim 1, wherein the reflectivesurface is defined on an exterior surface of a transparent opticalsubstrate.
 4. The optical element of claim 3, wherein the transparentoptical substrate is configured to define a refractive optical surfacethat is situated on the optical axis and is configured to converge anincident light flux toward the sample surface.
 5. The optical element ofclaim 1, wherein a curvature of the reflective surface is aspheric. 6.The optical element of claim 5, wherein the curvature of the reflectivesurface corresponds to a conic section.
 7. The optical element of claim6, wherein the conic section is a parabola.
 8. The optical element ofclaim 1, wherein the sample surface is situated optically proximate afocus of the reflective optical surface.
 9. The optical element of claim1, wherein the reflective optical surface extends so as to receive alight flux in a solid angle of at least about 1.5π steradians.
 10. Theoptical element of claim 1, wherein the reflective optical surfaceincludes a probe aperture that defines a probe path that couples thesample surface and the probe aperture.
 11. An optical assembly,comprising: a lens situated on an optical axis, the lens configured toreceive a primary light flux and converge the primary light flux towarda sample plane; a concave reflective surface situated along the opticalaxis with respect to the sample plane so as to direct a secondary lightflux produced in response to the primary light flux back towards thelens and along the optical axis.
 12. The optical assembly of claim 11,wherein the lens substantially occupies an exit aperture of thereflective surface.
 13. The optical assembly of claim 11, wherein thereflective surface is configured to substantially collimate thesecondary light flux.
 14. A catadioptric objective, comprising atransparent refractive substrate having a refractive surface configuredto direct an input optical beam toward a target region and a reflectivesurface configured to receive at least a portion of a light fluxpropagating from the target region produced in response to the inputoptical beam.
 15. The catadioptric objective of claim 14, wherein thereflective surface includes a reflective coating.
 16. The catadioptricobjective of claim 14, wherein the refractive surface is a surface ofrotation.
 17. The catadioptric objective of claim 14, wherein therefractive surface is situated on an optical axis and has a positivecurvature with respect to the input beam, and the reflective surface issituated on the axis and has a negative curvature with respect to theinput beam.
 18. The catadioptric objective of claim 17, wherein thereflective surface is an aspheric surface.
 19. The catadioptricobjective of claim 14, wherein a curvature of the reflective surface isa conic section.
 20. The catadioptric objective of claim 19, wherein theconic section is a parabola.
 21. The catadioptric objective of claim 14,wherein a center of rotation of the refractive surface and a center ofrotation of the reflective surface are situated on an optical axis. 22.The catadioptric objective of claim 21, further comprising an exitsurface that truncates the reflective surface and is situatedsubstantially at a focal plane of the refractive surface.
 23. Thecatadioptric objective of claim 21, further comprising an exit surfacethat truncates the reflective surface is situated substantially at afocal plane of the reflective surface.
 24. A transparent opticalsubstrate, comprising, along an axis: an input surface defining an inputaperture and configured to receive a light flux; a reflective surface;and an exit surface defining an exit aperture configured to receive thelight flux from the input aperture, wherein the reflective surfaceextends from the input surface to the exit surface and the exit surfaceis situated optically proximate a focus of the reflective surface. 25.The optical substrate of claim 24, wherein the input and exit surfacesare substantially planar.
 26. The optical substrate of claim 24, whereinthe input surface is configured to increase a convergence of thereceived light flux.
 27. The optical substrate of claim 24, furthercomprising an optical coating situated at the reflective surface andconfigured to enhance a reflectivity of the reflective surface.
 28. Animmersion objective, comprising: a reflective surface having an inputaperture configured to receive a primary light flux and an exit apertureconfigured to deliver the primary light flux to a sample, wherein thereflective surface, the input aperture, and the exit aperture aresituated on an optical axis, and an internal volume is defined by theinput aperture, the exit aperture, and the reflective surface; and anoptically transmissive barrier coupled to the reflective surface andconfigured to divide the internal volume into an illumination volume andan immersion volume.
 29. The immersion objective of claim 28, whereinthe exit aperture is situated at a focus of the reflective surface. 30.The immersion objective of claim 28, further comprising a hollow shellthat defines the input aperture, the exit aperture, and the reflectivesurface.
 31. The immersion objective of claim 28, wherein the immersionvolume is filled with an immersion fluid.
 32. The immersion objective ofclaim 28, wherein a portion of the reflective surface that bounds theinternal volume is defined on an optically transmissive substrate. 33.The immersion objective of claim 28, further comprising a lens situatedat the optically transmissive barrier and configured to converge asubstantially collimated light flux incident from the input aperture atthe exit aperture.
 34. The immersion objective of claim 28, wherein thereflective surface includes a probe aperture in a portion of thereflective surface that defines the immersion volume, wherein the probeaperture is coupled to the immersion volume.
 35. The immersion objectiveof claim 34, wherein a probe aperture axis that extends from the probeaperture intersects the exit aperture substantially at the optical axis,wherein an angle between the probe axis and the optical axis is lessthan about 60 degrees.
 36. A microscope objective assembly, comprising:a first reflector and a second reflector situated along an axis, thesecond reflector configured to reflect to the first reflector anillumination flux received from an aperture defined in the firstreflector and wherein the first reflector directs the reflectedillumination beam so as to substantially converge at a sample location;and a reflective collector configured to receive a secondary light fluxproduced in response to the converged illumination beam, wherein thereflective collector includes a central aperture.
 37. The microscopeobjective assembly of claim 36, wherein the first reflector and thesecond reflector provide a positive optical power and a negative opticalpower, respectively, to the illumination beam, the reflective collectorhas a conic constant of between about −0.85 and −1.0.